Space-current device



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ov. 2,' l934 N, c, PEARcY y SPACE CURRENT DEVICE Original Filed Feb. 20, 1924 mvENToR /Vo C. Panni-y.

ATTORNEY WITNESSES a Patented Nov. 20, 1934 UNITED STATT-:s

PATENT oi-FlcE SPACE-CURRENT DEVICE original application February'zo, 1924, serial No.

694,028.. Divided and this application July 24, 1934, serial'No. 736,697

2 Claims.

Thisapplication is a division of my copending application Serial Number 551,411, filed July 17, v1931, for Space-current devices which is in turn a division of Serial Number 694,028, filed February My invention relates to space-current devices and it has particular relation to devices of the above designated character utilizing indirectly heated cathodes for gas illled rectifiers.

One object of my invention is to provide a space-current device having an improved cathode capable of maintaining a large electron emission and which is capable of withstanding the positiveion bombardment met with in commercial gasfilled rectiers.

Another object of my invention is to provide a gas filled rectifier that may be ei'ciently used in connection with large rectified currents.

According to my invention, I provide a gas lled y rectifier having a cathode which is either directly or indirectly heated and employs as an electronemitting element a fused mass of Anickel and thermionically active oxides, such as barium oxide, strontium oxide and calcium oxide. I thus obtain an electron-emitting element which has very high emissivity and maintains about the same characteristics for 'avery long time, far exceeding the life and the performance of the ordinary tungsten electrodes.

My invention is particularly well adapted fo use in connect/ion with thermionic tubes ofthe type described in the copending application of H. M. Freeman and W. G. Wade, Serial No. 4 611,263, iiled January 8, 1923 and assigned to the Westinghouse Electric and Manufacturing Company.

Other objects of my invention consist in the provision of new methods of making an improved space-current devicejand thermionic cathodes of 40 the abovel designated character. With the foregoing and other objects in view, my invention consists in the details of structure and methods of construction described hereinafter and illustrated in the accompanying drawing, wherein Figure 1 is a View in elevation of a rectifier according to my invention with a portion of the front wall broken away to show the inner structure.

Fig. 2 is a similar view illustrating a modifica- (Cl. Z-27.5)

`Fig. 5 is a diagrammatic view of a rectifiersystem embodying my invention.

By way of illustration, I have shown my cathode applied to a rectifier, although it is susceptible of other uses, as in three-electrodetubes. ring to Fig. 1, a rectifier comprises an evacuated envelope 1 having mounted therein an anode 2 vand a cathode structure 3 which is supported in a familiar manner by a rod 4 and a terminal strap 6 of metal constituting the electric connection to the cathode. The envelope is preferably lled with a small quantity of inert gas such as argon or mercury vapor for securing an atmosphere of molecules capable of carrying a discharge between the cathode and the anode at relatively low pressures, as is well known in the art.

The principal feature of my invention is the construction of the cathode 3, one modification of which is shown in Fig. 3 and which comprises a cylindrical member 8 of a refractory insulating material, such as Zircon or porcelain, having a heating element 9, of U shape, extending through longitudinally disposed perforations therein. The heatingelement is supplied with two terminal wires l1, one of which may be connected to the terminal strip 6 leading to the cathode surface. Surrounding the cylindrical member 8 and in close contact therewith is a sleeve 12 comprising a fused mass ofl metal and oxides of the alkaline earth metals, the sleeve constituting the electron-emitting element 'l of thev cathode. In making the sleeveI prefer to use, nickel on account of its `relatively low melting point and since it gives very good performance. As electron-emitting oxides, I employ the oxides of barium, strontium and calcium which are fused or sintered with finely divided nickel particles to form a homogeneous mass and which are firmly held to the nickel, assuring a long life and producing a cathode capable of standing the positiveion bombardment and maintaining substantially constant performance throughout the life ofthe rectifier. The cathode just described is much superior to a cathode structure wherein the high electron emissivity is obtained by dipping a metallic surface in a proper solution for obtaining an oxide coat, since the coat which is obtained by such dipping is destroyed, after a relatively short time, by the bombardment of the positive ions and Refer- 6 the dipping process. v

One of the novel steps of making an improved cathode according to my invention involves the intimate mixture of the finely pulverized compounds of the alkaline earth metals, and also the employment of finely pulverized nickel. To obtain the finely divided nickel I take nickel oxide4 and heat the same in an atmosphere of hydrogen at approximately 600 C. until the oxide is reduced to metal. To obtain the intimate mixture of compounds of the alkaline earth metals used in my improved method of making the cathode, I dissolve amixture a barium nitrate, strontium nitrate and calcium nitrate in water and. precipitate the carbonates of the above-named metals from the solution so obtained, by adding a solution of ammonium carbonate. The precipitate thus obtained, when filtered, washed and dried, constitutes a very intimate mixture of the carbonates of the three metals.

The above-described method of obtaining a pulverized mixture of the compounds of the three metals consists, in general', in dissolving soluble salts of the metals which are to be mixed in a suitable solvent and precipitating from the solution insoluble salts of the metals. In a practical instance, I have used the following proportions:

Per cent Barium nitrate f 40 Strontium nitrate 26.67 Calcium nitrate 33.33

I regard the above-described step of mixing the ingredients constituting the lelectron-emitting element of the improved cathode as instrumental in securing the performance of the same and desire that the same be regarded as a distinct feature of my invention.

'I'he nickel particles and the carbonates of the alkaline earth metals are lmixed together and moistened with'a suitablebinder such as water yor an acid, such as acetic acid, for example, which reacts -with a portion of the oarbonates and forms a soluble salt of the alkaline earth metals.4 I thus obtain a plastic mass which is extruded through a suitable die into the desired shape. I may form rods or slabs, or in case of a cathode employed in a rectifier as shown in Fig. 1, I form a cylindrical sleeve which fits over thefcylindrical refractory element 8.

The extruded body is dried and then heated in an atmosphere of hydrogen to `a temperature sufficient to sinter the materials constituting the same. In a practical instance where I use the proportions of the ingredientsnamed above, I bring the heated mass to atemperature -of approximately 1000 C.

After removing the sintered body thus obtained from the furnace, the same is ready to be assembled in the space-current device, the carbonates` of the alkaline earth metals forming with the nickel, a homogeneous, mechanically strong mass.. The carbonates may be reduced to oxides either during'the sintering process or at any time afterwards, preferably when treatingthe tube on the pump in a Well-known manner,

after the same has been completely assembled.

According to another method, I may form the plastic mass ofthe nickel particles and the carbonates of the alkaline earth metals into a sleeve surrounding the heater member 3 and introduce the composite body thus obtained into the furnace for carrying out the sintering process as described hereinbefore.

necessary to make an oxide coated electrode by The connection of the electric lead'orstrip'B to the cathode may be madeeither by securing the strip to the cylinder 12 by any mechanical means such as a wire 13 wound around the cylinder or by welding the strap directly to the surface of the cylinder, or by a combination of both methods. The cylinder 12 is secured at its upper end to the holding rod 4, by a strip 14 similar to strip 6 or by any other suitable means.`

Rectiflers and tubes made with cathodesobtained in the manner described hereinabove operate very successfully and give larger cur.- rents at lower voltages than are possible with any other cathode construction of which I am aware. In particular I am able to operate such cathodes in rectiilers without maintaining a current in the filamentary heater 9, as the current therethrough may be turned oif as soon as the current flow'through the rectifier is initiated, the bombardment of the positive ions being sufficient to maintain the cathode at a temperature where the same is in an electron-emitting state.

An operative unit of the last described character is shown in Fig. 5. A double-wave rectifier is provided with an improved cathode 3a made according to my invention and two anodes 2a leading to a supply transformer 17a. A directcurrent load device 15 is connected between the active coat 16 of thecathode and the middle terminal 17 lof the transformer 17a, The heating filament 18 of the cathode is supplied from an auxiliary transformer winding 19 through a l switch 21 which is electromagnetically controlled by the current flowing to the direct current load. The switch is closed when the load current is below a certain value and opened when the current exceeds thatvvalu at which the positiveion bombardment maintains the cathode in an electron emitting state. 1

' While a somewhat similar operation of rectiflers has been described heretofore, I am not aware of any practical construction utilizing a cathode which embodied the oxides of the alkaline earth metals and which was capable of prov ducing a rectifier carrying a large amount of current'at the low pressures utilized in my devices.

For instance, the electron emission which is obtained by means of my improved cathode is many times larger than the electron emission possible when the cathode consists of a metal bead such as tungsten.

In Fig. 4, is shown a modification of my invention, the refractory cylinder 8 being coated with nickel or other metal previous to the assembly in the electron-emitting sleeve 12, the whole body being assembled when hot so that the nickel coat is plastic and forms an intimate connection between the sleeve 12 and therefractory member 8, facilitating the ow of heat from the heater member to the sleeve and expediting the step of initiating the' operation of the rectier.

I have found that the plastic mass of the nickel and alkaline earth metals may be extruded through a suitable die to form a lament similar other constructions. A tube made according to the last named method is shown in Fig. 2.

A distinct advantage of the filaments and cath- 4odes made according to my invention is that the same are operated at relatively low temperatures and they are less subject to destruction than cathodes operated at high temperatures.

The directly heated cathodes of the prior art had a disadvantage that is obviated by my indirectly heated cathode construction. In general, these cathodes of the prior art have a long wire for large cathode| area with the result that one part of the wire is more negative than another coating tends to evenly distribute the discharge over the cathode surface.

My invention also provides a practical construction for larger currents than have hitherto been practicable with the gas rectifier, having directly heated cathodes. Large currents requirea large cathode surface and a high temperature. With the directly heated cathodes of the prior art,lthese requirements meant a small diameter wire in order to provide the -high temperature and an extremely long one to provide Vthe large surface.

so l

However, the practical dimcultiesbfsupt'i such along wire and maintaining the slender wire under bombardment have limited the size of the prior gas lled rectiers.

Another diiiiculty is that large currents under very small voltage pressure are very unstable and difiicult to regulate in a long wire subject to a discharge. y I

With my construction of an indirectly heated cathode, however, the electron emitting surface and the connections thereto can be designed most emciently for the discharge or rectied current and the heater circuit can be designed for the most eiiicient supply of heat. If a small diameter heater Wire is used, it is protected from the discharge by the refractory insulating material. The size of this refraction core makes the electron emitting surface surrounding it, inherently large.

I do not desire to be limited to 'the precise process and details of construction described hereinbefore, but I desire that the appended claims shall be givenvtlie broadest contruction consistent with the prior art.

I claim as my invention: 1

l. An electrode of the class described comprising a solidified` coherent mixture of finely divided base metal and an allrali or alkaline earth metal oxide `fused on the surface.

2. An electrode of the class described comprising a mixture of finely divided base metal and finely divided alkali or alkaline earth metal solidined into a coherent mass and fused on the surface.

NOAH C. PEARCY. 

